Chair base



FIG. 1 is a perspective view of the chair backrest showing my newdesign;

FIG. 2 is a front elevation view thereof;

FIG. 3 is a rear elevation view thereof;

FIG. 4 is a left side elevation view thereof;

FIG. 5 is a right side elevation view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof.

CLAIM The ornamental design for a chair base, as shown and described.